The invention relates to apparatus for moving parts in machine tools, for example, to apparatus for moving grinding wheels or like tools in grinding machines. More particularly, the invention relates to improvements in apparatus which are used to move a mobile support for one or more selected parts (such as grinding tools) relative to a predetermined axis to and from one or more predetermined angular positions in which the mobile support is in frictional surface-to-surface engagement with a stationary support. By way of example, the apparatus of the present invention can be used with advantage to pivotally support the head of a grinding wheel carrier in a surface grinding machine.
The carrier of tools in a surface grinding machine must be pivotably mounted on its support (for example, on a cross slide) when the grinding machine is designed in such a way that the carrier of grinding tools supports two or more grinding tools, e.g., a first tool for internal grinding, a second tool for external grinding of rotary cylindrical parts, and a third tool for flat grinding. Such plural tools are preferably mounted at several sides of a substantially turret-shaped carrier which is indexible about a predetermined axis so as to move a selected tool to the grinding station. The same holds true if a substantially turret-shaped carrier supports several identical grinding tools (for example, grinding wheels) which are to be moved to the grinding station in a predetermined sequence. The means for indexing the carrier normally comprises a motor. It is often necessary to change the position of the carrier for grinding tools in different directions, for example, while grinding a conical surface or during so-called angular infeed grinding. At such times, it is necessary to move a selected grinding tool to one or more different angular positions with reference to the workpiece which is held between a headstock and a tailstock or in any other suitable way.
The art of machine tools provides a variety of auxiliary apparatus which facilitate movements of tool-bearing supports relative to stationary supports. For example, it is known to mount the tools on a carriage which is movable relative to a table or a similar stationary support. Reference may be had to German Pat. No. 719,863 which discloses an upright grinding machine with an arm which must be mechanically lifted prior to pivoting so that it is separated from a base plate with which it is in frictional engagement due to its own weight while the selected tool is located at the grinding station.
German Offenlegungsschrift No. 1,963,354 discloses another mode of reducing friction between an upper table and a lower table or the like. The object of the invention which is disclosed in this publication is to ensure that the upper table can be moved to a selected position with a minimum of friction and that such upper table can be form-lockingly held in its selected position with reference to the lower table. The publication is particularly concerned with elimination or reduction of the so-called stick-slip effect which is a phenomenon entailing adherence of the movable part to the stationary part, especially during the initial stage of movement of the movable part relative to the stationary part. The publication proposes to establish between the two tables an air cushion before the upper table is set in motion relative to the lower table.
A different mode of reducing friction between a stationary and a movable support in a machine tool or the like is disclosed in German Auslegeschrift No. 1,220,228. The machine of this publication comprises a hydrostatic bearing which is interposed between the stationary and movable supports.
The problem of accurately and predictably moving a mobile support relative to a stationary support is especially acute if the movable support is the carrier of tools in a grinding machine. This is due to the fact that the distribution of weight around the axis of rotation of the movable support is not necessarily uniform, e.g., when the center of gravity of the movable support is not located on the axis about which the movable support is to be indexed. Furthermore, the axis about which the movable support is indexed is often inclined so that the movable support tends to overhang in cantilever fashion. Additional problems arise when the movable support is installed in a grinding machine which is designed to finish workpieces with a high or utmost degree of accuracy.